1. Field of the Invention
This invention relates to an ultrasonic blood flow imaging method and apparatus, and more particularly to such a method and apparatus capable of two-dimensionally displaying the velocity distribution of blood flow in a body to be examined.
2. Prior Art
There are known ultrasonic imaging methods wherein an ultrasonic wave beam is directed into an object to be examined and an image is formed using the reflected echo produced as a result of differences in acoustic impedance within the object. These methods are advantageous in that they make it possible to observe the interior of an object without adversely affecting its structural make-up and are therefore used, for example, in diagnostic examination of afflicted tissues and organs in humans. These methods have been applied in ultrasonic diagnostic apparatuses and the like.
A more sophisticated utilization of ultrasonic waves developed in recent years is the pulse-doppler method, wherein the doppler effect that arises when an ultrasonic pulse beam strikes blood flow in a body to be examined is used to determine the velocity of the blood flow. This method has been successfully applied in, for example, blood flow velocity meters.
Although this velocity distribution has conventionally been displayed as a black-and-white image, it has, with the devices used for this purpose, been difficult to represent the two-dimensional distribution accurately. Thus, proposals have been made to improve the situation by use of color imaging method.
The principle of the conventional imaging method is shown in FIG. 1. When the signal representing the movement of the blood flow varies periodically as shown in the figure, six colors (a,b,c,d,e and f) are assigned one each for six velocity bands in the positive direction and six colors (h-m) are assigned for six velocity bands in the negative direction. In this way, colors can be displayed to correspond to the velocity distribution. This conventional system has the advantage of making it possible to judge the velocity distribution of a glance from the colors displayed. On the other hand, however, although this conventional method provides a very clear picture when the velocity of the blood flow remains constant, it has the disadvantage that the colors mix to produce a picture that is hard to read when the velocity and the direction of the blood flow rapidly varies, especially in blood flow of the heart. This method is also disadvantageous in a case like that shown in FIG. 2 where the change with time is not regular and local portions exhibit large deviations within the overall blood flow movement, since in such cases the colors mix throughout the picture making it difficult to read the picture and almost totally impossible to interpret those portions where the positive and the negative directions of blood flow are intermingled.